A telemetry system is set forth in the present disclosure which is particularly useful in well logging tools. It is particularly intended for use with one or more logging devices supported on a logging cable and enclosed within a sonde wherein the logging tools provide logging data at ever increasing data transfer rates. In particular, it is a telemetry system which will successfully operate on a single conductor (hence the term monocable) within the logging cable wherein the single conductor carries other signal and power transmission on the cable.
When first introduced, downhole logging tools performed measurements which transmitted signals to the surface as analog signals. As time passed, more sophisticated systems came into being including AM, FM, PCM, etc. Analog capacity reached the equivalent of about 4,000 bits per second using a PPM systems. Analog transfer, however, has become obsolete as digital computers have come to the front in execution of surface data processing. The present advanced logging systems use QPSK or three level duobinary coding. This has accomplished some bandwidth reduction by a factor of two fold. Cable parameters must be carefully determined and carefully monitored because analog equalizers are normally used to remove cable distortion. Obviously, not every cable is equally well made, and cables do vary in their transfer function so that cables cannot always be properly matched with telemetry systems. The present disclosure is directed to a telemetry system which can function without requiring extraordinary cable quality and will function notwithstanding variations in cable transfer characteristics.
In very general terms, the telemetry system has a downhole transmitter connected to a surface receiver. There is however the transfer function of the cable which inevitably distorts and attenuates the signal transmitted along the cable. The received signal must be processed so that the data of interest can be recovered without errors. The problem is made even more difficult because the monocable is often used for the transfer of other data. Data from the surface can be sent downwardly on the same monocable and must be accommodated so that instructions for operations of the downhole logging tool can be obtained. It is also common to place a DC voltage on the cable so that electrical power is transferred to the tool for operation of various electrical components in the tool. With this backdrop, the uplink data must be transmitted along the monocable subject to variable distortion, and transmission occurs in the presence of other signals interposed on the same current conductor.
The monocable is typically comprised of a single conductor with a shield or alternate conductor serving as ground. Data is created in the logging tool and has the form of a sequence of binary symbols. The logging tool telemetry apparatus will be described as converting the data from the logging tool into a selected format such as NRZ data, a mixed sequence of binary zeros or binary ones. The data is preferably transmitted at a particular clock rate. The downhole system preferably incorporates a scrambler provided with the NRZ data stream which distributes the ones and zeros in a pseudorandom sequence and also assures level transitions while avoiding forming a long fixed value. This makes it easier to operate the AGC (automatic gain control) amplifier and clock recovery circuit at the surface as will be described. The present apparatus first converts a stream of NRZ data bits to four level data by converting each successive two bits of NRZ data into one four level data symbol. The four level signal is then converted into a seven level duobinary-encoded signal that requires half the bandwidth of the four level signal and a fourth of the bandwidth of the original NRZ signal. As the type and diameter of the cable permit, the bandwidth for data transmission along the monocable increases. Even so, there is a limit to the maximum data rate that can be transmitted on any particular monocable given a particular noise environment. The modulation scheme described herein allows that maximum data rate to be more closely approached than previous modulation schemes. This type of data encoding, which compresses the required signal bandwidth, allows a higher rate of data transfer than any previous system would allow for any particular monocable.
The logging cable (defined as a pair of conductors) extending from the surface is a form of transmission line. The cable has a certain transfer characteristic. In fact, the monocable is a transmission line which has a limited bandwidth. If the data transfer rate is increased, cable limitations cause serious data degradation. One result of limited band width is the fact that signal output has reduced harmonic content so that the output is severely distorted and is primarily an analog signal. Adjacent digital symbols contribute to intersymbol interference when transmitted along the cable. As the distortion and interference increase and signal amplitude decreases, limits in data transfer capacity are encountered. In the present apparatus, seven pulse levels are used so that each level of the seven represents two bits of data. By using seven levels, bandwidth efficiency is increased and the data transfer rate is enhanced. The theoretical bit rate in this approach is in part limited by the permitted signal to noise ratio for quality signal transmission. The present system thus uses seven levels of digital data, the levels centered at zero and includes three symmetrical levels above and below zero. The data in encoded in a particular way (called multilevel correlative coding) to achieve the seven levels while limiting the transmission bandwidth of the signal. In summary, the bandwidth efficiency is four times that of a NRZ system using amplitude modulation. Use of seven level encoding permits correlation between adjacent data bits. Assume that the four state symbols can be encoded to the seven levels, leaving three of the states unused. The "surplus" states are selected to encode the four state symbol plus some aspect of adjacent symbols and hence assures improved data recovery by adjacent symbol correlation.
Appropriate coupling circuits separate the uplink telemetry system, downlink telemetry system, and DC power supply connections for operation on the monocable. The present disclosure is thus directed to a downhole sonde supported data encoding system. It also discloses a data receiving system which is installed at the surface. The equipment at the surface must reverse distortion that was created by the cable on the transmitted signal. If the cable were precisely fixed and unchanging, the nature of the distortion could be permanently known, but this is not the good fortune of operation. Rather, the distortion is variable. The distortion is overcome in a manner to be described below by use of an adaptive transversal filter equalizer. The adaptive transversal filter equalizer automatically adjusts its transfer function to correct for a variable amount of cable distortion which distortion must be assumed to vary dynamically.
A seven level encoding system is set forth, thereby enabling a single seven level symbol to represent two symbols decoded from NRZ binary. The seven levels make decoding more difficult, but it enables the transmission of far more data without increasing the required bandwidth in the monocable. Data recovery is limited by the signal to noise ratio. Accordingly, the downhole telemetry equipment converts the data from typical NRZ binary data into an amplitude modulated (AM) seven level duobinary set of symbols which are then filtered to limit the bandwidth of the transmitted signal and which is thereby converted into an analog signal. That signal is then amplified by a power amplifier for application to the monocable. Appropriate coupling circuits separate uplink transmitter data, downlink received data and poweer for opperation of the logging tool. The transmitted analog signal is propagated up the logging cable to the receiver. There, an uplink receiver having a filter separates the signal from downlink transmitted data and converts the received or uplink signal into a suitable signal for recovering the original data. The receiving apparatus at the surface includes an automatic gain control amplifier (AGC), a related clock recovery circuit to reconstruct the clock signal in the received uplink signal, an analog to digital converter and an equalizer and slicer circuit. The equalizer circuit in conjunction with the slicer circuit converts the digital signals into the encoding levels originally involved (seven levels in the preferred system). A descrambler circuit is included at the surface to reverse the effect of the downhole scrambler. Most sondes will support at least two different logging tools which form two different data streams. Assuming that a multiplexer is used in the sonde to transmit data from two or more tools, the data is transmitted in specific data frames. This is a time multiplexed sequence which is sorted by computer at the surface. So to speak, a demultiplexer is included at the surface by sorting time frames, and the several output data are then delivered for data processing and/or storage in typical recorders which record the data as a function of depth in the well borehole. In the preferred embodiment, the two or more tools in the sonde furnish data for transmission in response to surface originated signals; in that arrangement, data frames are interwoven, enabling transfer of two or more data streams. At the surface, the two or more data streams must be sorted out and in this regard, the recovered signal may require demultiplexing to separate multiple transmitted signals.
Emphasis should be focused on the equalizer and slicer. The equalizer is provided with digital values which ideally represent the levels of the encoded input, or seven levels in the preferred embodiment. However, because of unknown and variable distortions arising from variations in cable temperature and length, the input to the equalizer is not precisely at the seven levels originally transmitted. The received signal, after has error due to noise, phase shift, temperature variation, etc. Consider a seven level output system where 2.0 units amplitude is one of the digital levels. If the output of the equalizer is 2.18, slicing must occur to reduce that value to 2.00. In other words, slicing recreates levels matching the transmitted levels. Even where errors arise from the distortion to the signal occurring during cable transmission, such errors are removed by the present apparatus without regard to the precise transfer characteristics of the cable.
With the foregoing in view, the present apparatus is very briefly described as a logging tool telemetry system which transmits a multiple level signal outputting an analog signal after transmission along a monocable in the logging cable to surface located uplink telemetry receiving apparatus. The signal is processed through an AGC amp, is digitized by an ADC, the clock synchronization in the signal is recovered, and the output is then passed through an equalizer and slicer. The output is delivered to one or multiple recorders after data processing for recording thereby, and such data is recorded as a function of depth in the well borehole. The system operates substantially free of different or variable transmission characteristics of the monocable.